The vibrating motions we have discussed so far have taken place in ideal systems that oscillate indefinitely under the action of a linear restoring force. In all real mechanical systems, forces of friction retard the motion, so the systems don’t oscillate indefinitely. The friction reduces the mechanical energy of the system as time passes, and the motion is said to be damped

Examples of damped oscillations are a swinging bell (Figure 49a) and an object attached to a spring and submersed in a viscous liquid. The swinging bell left to itself will eventually stop oscillating due to damping forces (air resistance and friction at the point of suspension). An object attached to a spring and set in oscillation in a viscous liquid (Figure 49b) will come to rest due to the drag force it experiences in the liquid.